Thermal converters are the most accurate instruments known for the measurement of ac voltage and current at frequencies ranging from low audio frequency on up to 1 GHz. Multijunction thermal converter structures are most conducive to a wide range of uses. These are frequently used in very high accuracy ac-dc difference metrology, because they exhibit very small ac-dc differences, provide a good square law response, and high output emfs. In one form or another, such multijunction thermal converters are employed to generate primary standards for the National Institute of Standards and Technology (NIST) for ac-dc difference, ac voltage and current, and ac power and energy calibration services.
The basic MLF-MJTC structure includes a heater element, a plurality of cooperating thermocouples connected in series, i.e., one or more thermopiles, and electrical wiring connecting the MLF-MJTC to an external circuit or known types of voltage, current and power measurement devices. In addition to the basic MLF-MJTC structure, one or more resistors connected to heater element to receive input voltage and/or current. Preferably, the thermopiles are electrically insulated from the heater element to ensure high response accuracy, i.e., so that an unknown signal will not flow into the circuit which is used to monitor the thermopiles (or even into the thermopiles themselves) as this would produce an error.
In the past, thermal converter structures have typically included fine wires of different conductive materials to form sets of thermocouples carefully located with respect to each other and the heater element. There are significant problems in physically handling the very fine wires that are combined to make such thermocouples and in soldering or otherwise affixing them to each other and to the rest of the structure. An example of such an early structure is disclosed in U.S. Pat. No. 839,985 to Bristol, titled "THERMOELECTRIC GENERATOR".
More recently, a variety of thermocouples and thermal converter structures have been designed for specific applications through the use of thin film and thick film technologies.
Regardless of which physical form is selected, i.e., whether wires or thin films deposited by any known technique are employed, the principal physical phenomenon being exploited depends on the fact that when different thermal electric materials are joined at their spaced-apart ends, with the junctions located in regions at different temperatures, the difference in the internal electron structures of the two materials causes a voltage difference to exist between the junctions. This voltage difference, which the typical thermocouple is constructed to use, is temperature-dependent and is known in the art as the "Seebeck effect". If the materials used are conductors or semiconductors, a current will flow through the thermocouple and may be extracted by connection to elements of an external circuit, typically through contact pads. When a number of thermocouples are connected in series, to enhance the rather small voltage differences generated in the individual thermocouples under certain circumstances, the plurality of thermocouples is referred to as a "thermopile". Thermopiles may be used for a variety of purposes, e.g., to determine a voltage or current, to generate electricity from sources of heat such as sunlight received by a solar collector system, or to actuate protective or sensing devices.
In light of the variety of uses to which a thermocouple or a thermopile may be employed in a device to convert energy of one form into another, e.g., heat or incident electromagnetic radiation into a current, it is important to clarify the use of the term "converter" as used in the present application. In the following description of the present invention, the term "converter" is used to refer to conversion of each of an ac and then a dc signal to heat, and in each case to generate from the heat corresponding output electrical signals which are monitored. In essence, the devices and the methods described more fully hereinbelow relate to generating electric energy from a very precisely controlled source of heat energy, this being accomplished by the provision of a very precisely formed and operated electrically-powered heater element and selectively disposed sets of thermocouples with their hot junctions heated by the heater.
Very precise standards for determining a voltage or current over a wide range of frequencies for ac-dc and RF-dc applications are necessary for an MLF-MJTC. To meet this need, an MLF-MJTC requires physical ruggedness, ability to withstand stresses associated with thermal cycling and in-use exposure to a wide range of temperatures locally, and an error-free structure, e.g., minimization of capacitances or inductances developed between the heater and thermopiles.
Such devices may be used to measure the power or calibrate the energy of radiated signals from optical sources, e.g., infrared sources or lasers. This operation relies on the physical principle that when radiation is incident on the invention a certain percentage of it is absorbed and the heater region of the invention is heated, thus producing a change in its emf output that is proportional to the incident energy. The measurement of this output can be used to determine the power or energy of the incident radiation.
Also, the measurement of vacuum, pressure or airflow can be accomplished with the present invention. This operation is permitted by the physical principle that the amount of heat lost by convection when operated at a constant power is proportional to the pressure and gas flow. The number of molecules striking the heated surface and leaving it per second is proportional to either the pressure or the flow of the gas over the surface. By measuring the temperature, which is inversely proportional to the thermal loss, which in turn is inversely proportional to either the pressure or gas flow, the pressure or gas flow can be measured. It is noted that these two properties are not measured at the same time. Consequently, pressure is measured when there is no significant gas flow and vice versa.
In order to maximize the accuracy of MLF-MJTC, it is necessary that the Thompson coefficient of the heater material be small, and the thermocouples be disposed uniformly and symmetrically on opposite sides of the elongated heater element. Low Peltier effect is also necessary for enhanced operation as exemplified by the present invention. To achieve this result the contact area between the heater and the contact pads should be on a good heat sink such as a silicon frame.
These and other related objects of this invention are described more fully hereinbelow, as are structural details of the preferred embodiments of this invention and methods of forming the same. Persons of ordinary skill in the art, upon understanding the following disclosure and the accompanying drawing figures may consider implementing obvious modifications and variations of this invention, and the claims appended hereto are intended to comprehend such variations of the explicitly disclosed embodiments.